BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 9. NUCLEIC ACIDS

9.4. Structure of nucleic acids

9.4.1. Primary structure of nucleic acids

The primary Structure of Nucleic Acids refers to the linear sequence of NUCLEOTIDES in the polynucleotide chains of RNA and DNA. The monomers in nucleic acid molecules are linked via an ester bond formed between the phosphate group of one mononucleotide and the 3'-OH group of the pentose moiety of another (the 3',5'-phosphodiester bond). Nucleic acid molecules from all Living organisms are unbranched polymers.

The ends of a polynucleotide chain differ in structure: one end contains a free phosphate group (the 5' end), while the other features a free 3'-OH group (the 3' end). Consequently, polynucleotide chains are polar and possess a defined 5' → 3' directionality (Fig. 9.5).

The primary structures of DNA and RNA differ in several aspects: 1) The pentose moiety—DNA nucleotides contain a 2'-deoxyribose residue, whereas RNA contains ribose. Notably, the presence of the hydroxyl group at the C-2' position of ribose makes the RNA molecule unstable in mildly alkaline environments, whereas DNA remains structurally unaffected under such conditions; 2) The composition of pyrimidine bases—DNA contains the pyrimidine thymine (5-methyluracil), whereas RNA contains uracil instead of thymine; 3) The presence of certain minor (modified) nucleotides.

Different Nucleic Acids vary in the number of mononucleotide residues per molecule, their nucleotide composition, and the specific sequence of these residues (essentially the nitrogenous bases, as the pentose-phosphate backbone is identical in all monomers). The qualitative composition and nucleotide sequence along the nucleic acid chains determine their unique structure and the functional individuality of DNA and RNA molecules.

The Introduction/19.html">Primary Structure of nucleic acids dictates their higher Levels of Organization, namely secondary and tertiary structures.